Power units

ABSTRACT

A free piston engine, or an opposed free piston engine, in which the power output is derived as a flow of pressurized hydraulic fluid. The engine piston, or each engine piston, coacts with a hydraulic pump which delivers hydraulic fluid into a constant displacement accumulator. The constant displacement accumulator is charged to a predetermined level on each power stroke of the engine piston, to store energy for effecting the recompression stroke. Pressurized fluid for driving a load motor is delivered in accordance with the opening of a pressure-responsive one-way valve, upon charging of the accumulator to its maximum level, so that the energy available for the recompression stroke of the piston is independent of load. In the case of an opposed free piston engine, a closed loop control system is provided for maintaining the engine pistons in synchronism.

States Patent [1 1 Fitzgerald Oct. 15, 1974 21 Appl. No.: 305,453

[30] Foreign Application Priority Data Nov. 18, 1971 Great Britain 53721/71 [56] References Cited UNITED STATES PATENTS Cadiou 417/364 X Panhard a 417/364 X Primary ExaminerWilliam L. Freeh Assistant Examiner-Richard Sher Attorney, Agent, or FirmRidout & Maybee 57 ABSTRACT A free piston engine, or an opposed free piston engine, in which the power output is derived as a flow of pressurized hydraulic fluid. The engine piston, or each engine piston, coacts with a hydraulic pump which delivers hydraulic fluid into a constant displacement accumulator. The constant displacement accumulator is charged to a predetermined level on each power stroke of the engine piston, to store energy for effecting the recompression stroke. Pressurized fluid for driving a load motor is delivered in accordance with the opening of a pressure-responsive one-way valve. upon charging of the accumulator to its maximum level, so that the energy available for the recompression stroke of the piston is independent of load. In the case of an opposed free piston engine, a closed loop control system is provided for maintaining the engine pistons in synchronism.

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sum "11 0F 14 FIG. 35

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FIG. 38

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POWER UNITS BACKGROUND OF THE INVENTION ber to which'a hydraulic accumulator is connected, the

hydraulic accumulator storing sufficient energy from each expansion stroke (or power stroke) of the engine to return the pistons and so effect the next compression stroke. Such power units have high potential utility, but in practice they have the disadvantage that the operating conditions of the engine are highly dependent on the load, since the energy available to effect the compression stroke on each cycle of the engine is a function of the useful energy being extracted from the unit. This makes such a unit very difficult to control. Furthermore, since the operating conditions of the engine are load-dependent, it is practically impossible with-known power units to operate the engine at maximum efficiency over a range of load conditions. This is a serious drawback, since the general adoption of such power units in the future, for vehicles and other uses, is likely to depend not only upon considerations of fuel consumption, but also upon their effect on the level of air pollution. In theory such a power unit, especially if it incorporates 'a compression-ignition engine, should have the advantage of a relatively clean combustion cycle as compared with conventional internal combustion engines. In practice, however, this advantage cannot be realized on account of the variable operating efficiency of the engine under variable load conditions.

It is an object of the present invention to provide a power unit of the kind referred to in which the engine operating conditions are substantially independent of load. This is achieved-by controlling the compression strokes of the engine piston, or pistons, in accordance with the charging of a constant displacement hydraulic accumulator.

A hydraulic accumulator is a device characterized by the provision of a storage chamber into which a hydraulic fluid may be admitted under pressure, and from which the fluid may be withdrawn, the storage chamber having a displaceable wall which is biassed in a direction such as to minimize the volume' of the storage chamber; the wall is displaceable against its bias by the admission of pressurized hydraulic fluid into the chamber, mechanical work thus being performed on the wall, which 'work is thereby converted into potential energy. As a general rule the'displacement of the wall is determined by a pressure differential on its opposite sides, the maximum displacement being variable in dependence upon the pressure of hydraulic fluid admitted into the chamber whatever the hydraulic fluid pressure, there being no predetermined maximum displacement. Such an accumulator may be referred to as a variable displacement hydraulic accumulator.

The expression constant displacement hydraulic accumulator as used herein means a hydraulic accumu- BRIEF SUMMARY OF THE INVENTION A power unit in accordance with the present invention, comprises a free piston engine having a reciprocatory power piston and a hydraulic pump piston coacting therewith, the pump piston working in a pump chamber having aninlet opening and a delivery opening, a constant displacement hydraulic accumulator connected to the pump chamber to receive fluid under pressure therefrom during the expansion stroke'of the engine and to return the fluid thereto during the cornpression stroke of the engine, a first one-way valve coacting with the inlet opening and operable in response to the discharge of the accumulator to a predetermined minimum level to admit low pressure fluid to the pump chamber during the compression stroke of the engine, and a second one-way valve coacting with the delivery opening and operable in response to charging of the accumulator to a predetermined maximum level to deliver fluid under pressure from the pump chamber during the expansion stroke of the engine.

In most applications, but not necessarily all applications, the engine will have a pair of opposed free pistons working in a common cylinder, and a pair of hydraulic pump units connected to the cylinder at opposite ends thereof, each pump unit comprising a pump piston coacting with a respective one of the free pistons of the engine.

Preferably, the engine is a compression ignition engme.

The invention also provides a power plant which comprises, in combination, an internal combustion engine having a pair of opposed free pistons working in a common cylinder, a pair of hydraulic pump units connected to the cylinder at opposite ends thereof, each pump unit comprising a pump piston coacting with a respective one of said free pistons and working in a pump chamber having an inlet opening and a delivery opening, a pair of constant displacement hydraulic accumulators connected respectively to the pump chambers to receive hydraulic fluid under pressure therefrom during the power strokes of the free pistons and to return the fluid thereto during the compression strokes of the free pistons, synchronizing means re-.

sponsive to the displacements of the pump pistons in said pump chambers for maintaining synchronous operation of the free pistons, a closed hydraulic power transmission system having a high pressure manifold connected to said pump chamber outlets and a low pressure reservoir connected to said pump chamber inlets, each pump unit in cluding a first one-way valve coacting with the inlet opening of the pump chamber and operable in response to the discharge of the respective hydraulic accumulator to a predetermined minimum level to admitlow pressure fluid to the pump chamber during the compression stroke of the free piston, and a second one-way valve coacting with the delivery opening of the pump chamber and operable in response to the charging of the respective hydraulic accumulator to a predetermined maximum level to deliver pressurized fluid from the pump chamber to the high pressure manifold during the expansion stroke of the free piston, at least one hydraulic motor connected in the hydraulic power transmission system to be driven by the working fluid therein, fuel injection means for delivering fuel to the engine cylinder, valve controlled means responsive to the gas pressure in the engine cylinder for controlling the fuel injection means in accordance with the cyclical movements of the free pistons, and control means for metering the fuel delivered by the fuel injection means. The outlet system includes a valve arranged to ensure that the fluid pressure in the high pressure manifold remains higher than the pressure in the constant displacement accumulators.

DESCRIPTION OF A PREFERRED EMBODIMENT One embodiment of the invention, as applied to a power unit and transmission system for a wheeled vehicle, will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a longitudinal section taken through the axis of the power unit;

FIG. 2 is a fragmentary top plan view of the power unit; 1

FIG.

FIG.

FIG.

FIG.

3 is a section on line 3-3 in FIG. I; 4 is a section on line 4-4 in FIG. I; 5 is a section on line 5-5 in FIG. 1; 6 is a section on line 6-6 in FIG. 1; FIG. 7 is a section on line 7-7 in FIG. 1; FIG. 8 is a section on line 8-8 in FIG. 6; FIGS. 9, 10, I1, 12 and 13 show details of a plate valve assembly shown in FIGS. 1, 6 and 8;

FIG. 14 is an underneath plan view of the detail shown in section in FIG. 6, without the inlet valve assembly;

FIG. 15 is a diagrammatic drawing showing leakage control of oil from the pump pistons of the power unit;

FIG. 16 shows a section on line 16-16 in FIG. 1;

FIG. 17 shows a section on line 17-17 in FIG. 2;

FIG. 18 shows a central vertical section through the fuel injector of the unit, the section being on line 18-18 of FIG. 19;

FIG. 19 is a partly sectioned side elevation of the fuel injector;

FIG. 20 is a section on line 20-20 in FIG. 18;

FIG. 21 is an unsectioned end view of FIG. 18;

FIG. 22 shows the outside of a spool valve element;

FIG. 23 is a section on line 23-23 in FIG. 18;

FIG. 24 is a section on line 24-24 in FIG. 18;

FIG. 25 is a section on line 25-25 in FIG. 18;

FIG. 26 is a sectional view of a valve connector adapted to be used with the fuel injector;

FIG. 27 is a part-sectional plan view of a control gear for starting the stopping the power unit;

FIG. 28 is a part elevation on line 28-28 in FIG. 27;

FIG. 29 is an unbroken plan view of the control gear;

FIG. 30 is an end elevation of the control gear viewed from the left in FIG. 27 with certain parts removed;

FIG. 31 is a sectional plan view on line 31-31 in FIG. 27;

FIG. 32 is a fragmentary view in the direction of arrow 32 in FIG. 27 with certain parts removed;

FIG. 33 is a part elevation showing the end view of a solenoid;

FIG. 34 is a partly broken away side elevation of a reversible hydraulic motor adapted for use with the power unit;

FIG. 35 is a section on line 35-35 in FIG. 34;

FIGS. 36 and 37 illustrate details of a planet gear bearing from opposite sides thereof;

FIG. 38 is a section on line 38-38 in FIG. 37;

FIG. 39 is a section on line 39-39 in FIG. 34 with certain parts removed, showing a planet gear without details of its teeth;

FIG. 40 is a schematic view of a gear wheel of the motor; and

FIGS. 41a and 4112 are schematic overall representations of the complete power unit and ancillary equipment;

FIG. 42 is a section on line 42-42 in FIG. 41;

FIG. 43 is a section on line 43-43 in FIG. 41;

THE POWER UNIT General The power unit comprises an internal combustion engine having a pair of opposed free pistons, a pair of pump units the pistons of which coact with the engine pistons, a pair of constant displacement hydraulic accumulators into which pressurized hydraulic fluid is pumped in accordance with the expansion strokes of the engine pistons, inlet ports and exhaust ports under the control of the pistons for admitting combustion air to, and exhausting combustion gases from, the engine, and valve operated fuel injection means actuated in accordance with the cyclical movements of the pistons to control the injection of fuel into the engine. The power output from the engine is a flow of pressurized hydraulic fluid, which in the present example is delivered from a pair of smoothing accumulators and used to drive bydraulic motors.

Arrangement and Mechanical Construction The mechanical construction of the power unit itself, and certain details of such construction, are illustrated in FIGS. 1 to 17, of which FIG. 1 best illustrates the general arrangement of the unit. Reference will now be made to these figures in particular.

At the heart of the power unit is a compressionignition engine comprising a single, water-cooled cylinder 101 having a ring of air inlet ports 102 and a ring of exhaust ports 103, and a pair of opposed free pistons 104L and 104R of equal mass. The pistons 104L and 104R are free to reciprocate within the cylinder 101,

and the overall design includes means to ensure that the pistons always move simultaneously in opposite directions and are also disposed symmetrically on opposite sides of a central position denoted by line 4-4 in FIG. I. A fuel injector 200 is bolted to the cylinder 101 at the central position, so that its nozzle 201 is positioned to inject fuel into the space between the opposed pistons at appropriate times, as will be described hereinafter.

Each end of the cylinder 101 is bolted to a respective one of two similar hydraulic accumulator-pump assemblies 105, 106. The assembly 105 (which will be described in detail, the assembly 106 being identical in construction) comprises a pump unit 107, a first, constant displacement hydraulic accumulator 108, and a second, high pressure or smoothing hydraulic accumulator 109, the assembly having a casing structure including a bulkhead 110 which is bolted to the cylinder end by bolts 111.

The pump unit 107 provides an internal oil-filled space or pump chamber 112, and houses a composite cylindrical or pump piston 113 which is a reasonably leak-free sliding fit in the bulkhead 110. The combined effects of the momentum of the pump piston 113, and the pressure in the pump chamber 112, ensure that the pump piston 113 is always pressed against the engine piston 1041,.

A groove 114 in the bulkhead'allows'oil leaking along the outer wall of the pump piston 113 to pass into a pipe 177 (FIG. 5) which conveys it to a float chamber 178. Within the float chamber 178 is a float 179, which on rising uncovers a drain hole 180 leading back to a vented reservoir 514, (FIG. 41) via a pipe X. A tube, 181 leading via a restrictor 182 to a hole 183 in the bulkhead 110 gives access to the air compression space 117. A valve 184 may be used'to open or close access to the-air compression space so that air may be extracted therefrom in order to form a vacuum with which to suck the pistons 104L and 104R into their starting position. The float 179 and drain hole 180 are preferably so dimensioned that the float will rise before its total submersion under any conditions of average pressure that may exist in float chamber 178. In any case after the engine is brought to rest, pressure in the float chamber 178 will fall so that oil leaking along the outer wall of the pump piston 113 will flow into it, and when the level is sufficiently high the float will rise and allow the leaking oil to flow down into a vented reservoir 514 (FIG. 41).

The pump piston 1 13 is in the form of a'hollow ram which defines an internal oil space and contains a heavy plunger 115 which is free to move back to a retaining screw 118, and forward to cover an oil flow restrictor 116 at the inner end of the ram; it will move in this manner under the impetus of its own inertia, pressing against the oil flow restrictor 116 at the inner end when the piston 104L is accelerating during the first part of its outward stroke and decelerating during the last part of its recompression stroke. The ram will be pressed against the retaining screw 118 during the last part of its expansion stroke and the first part of its recompression stroke. The restrictor 116 allows a controlled quantity of oil to pass into a hole shown by dotted lines 164 and on through a number of holes 165 into a groove 166 round the piston 104L for cylinder wall lubrication where the piston slides. A spring 118A contained within the retaining screw 118 urges the solid rod 115 inwards to close the restrictor 116 when the engine is at rest so that oil cannot escape through the restrictor at this time. a

The first, constant displacement accumulator 108 comprises a domed casing providing a stepped cylindrical internal surface 120. The domed casing houses a downwardly projecting cylindrical sleeve 121 in which a piston 122 is free to move axially up or down. The lower portion of said stepped cylindrical surface constitutes a cylinder communicating with the pump chamber 112 and locating a leak'free piston 123 which is free to move axially up or down. The pistons 122 and 123 define within the first accumulator a space 124 of.

variable volume which contains nitrogen or other gas under pressure. A second, oil-filled space 125 is situated above piston 122, to or from which oil may be admitted or withdrawn via a port 126 in the dome 127 of the casing. The piston 123 is formed with a flange 128 which is adapted to come to rest against a step 129 of said stepped'cylindrical surface 120 and to abut against the lower end of the sleeve 121, for limiting the downward and upward movements of the piston 123. Thus the piston 123 is constrained to move between lower and upper limit positions which determine the minimum and maximum charge levels ofthe accumulator, respectively. A sump 130 is formed by the piston 123, in which any oil that may leak into the space 124 will collect and from which it may be withdrawn via a duct 131.' This duct also serves for recharging the gas space 124 and to adjust its pressure; a suitable valve would normally be fitted into the duct 131.

It is necessary to ensure rapid establishment of inlet oil flow into the pump chamber 112 once the chamber pressure falls as the result of the flange 128 of accumulator piston 123 coming to rest against the step 129, while thepump piston 113 still continues its outward stroke. For this purpose, an inlet oil assembly 132 is provided to control the flow of oil through a springloaded plate valve 133. In this assembly: (I) the mass of the moving element of the valve 133 is kept reasonably low; (2) a diaphragm 132D, backed by a suitable gas such as nitrogen, contained in a space 132G, keeps to a minimum the mass of oil that must be accelerated on each cycle; (3) the cross-sectional area of the oil, perpendicular to its direction of flow, is large so as to keepthe oil velocity low; (4) the inlet oil in space 132E is raised to a fairly high pressure, which for example might in a particular instance be 150 pounds per square inch. When the pump piston 113 moves inwards into the'pump chamber l12,'the first accumulator is first charged to its full capacity and then oil is forced via a second automatic springloaded plate valve 134 into the second hydraulic accumulator 109. The details of construction of the second automatic plate valve 134, which is'essentially a high speed one-way valve, are illustrated in FIGS. 9 to 13. The first automatic plate valve may be similarly constructed. As illustrated in FIGS. 9 to 13, the valve comprises essentially a stationary valve element in the form of a grid, and a thin plate which is formed as a complementary grid, the thin plate being urged into contact with the stationary valve element by an array of compression springs. When the valve is closed, the grid elements of the thin plate close off the spaces between the elements of the stationary valve element; when the thin plate is displaced by a small amount, however, these spaces are opened simultaneously. Thus the valve opens substantially to its maximum extent with a minimal displacement of the movable valve element.

The second hydraulic accumulator 109, best shown in FIG. 6, is connected to the oil delivery opening of the pump chamber 112, which opening is controlled by the automatic oneway valve 134. The accumulator 109 comprises a domed casing housing a cylindrical sleeve 135 within which a piston 136 is free to slide axially up or down. The piston 136 defines within the sleeve 135 a space 137, which is filled with nitrogen or other suitable gas under pressure. A vent 138 (FIG. 17) for filling the space 137- leads into the clearance space that exists between the cylindrical sleeve 135 and the casing 109. When the pump piston 113 moves inwards it first charges the constant displacement recompression accumulator 108 by forcing the piston 123 up until the latter is brought to rest against the lower end of the sleeve 121, and oil then passes from the chamber 112 via the one-way valve 134 into an oil space 139 of the second accumulator 109, which has an outlet port 140 from which-the pressurized oil is supplied to the hydraulic load circuit.

Communicating with the air compression space 117 behind each of the engine pistons 104L and 104R is a thin spring-loaded air inlet valve 141 fitted in an entrance 142, and a thin spring-loaded air delivery valve 143 fitted in an'outlet 144; these valves admit air into the compression spaces 117 on the compression strokes of the pistons, and permit egress of air from the compression spaces on the expansion strokes of the pistons, respectively. The entrances 142 may be connected by flexible metal tubing to an air inlet filter, atmospheric air being filtered and admitted through the valves. in the preferred embodiment illustrated in the drawings, however, the entrances 142 are connected by a duct l45-to the outlet of an air compressor 146. The duct 145 may also include an air cooler. The compressed air from the outlets 144, after cooling if necessary, is conveyed via ducts 147 (shown broken away in FIG. 1) to an inlet 148 communicating via an air inlet manifold 149 with the inlet ports 102 of the engine cylinder (see FIG. 3). The inlet ports 102 and the ends of the air inlet manifold 149 arepreferably shaped so as to induce a swirling motion of the incoming air, for example as indicated by the arrows of FIG. 3. In the present embodiment the depth of the channel perpendicular to the-cross sectional plane of FIG. 3, tapers from the inlet 148 to the ends of the manifold so as to promote approximately the same air velocity throughout the manifold. This produces two elongated air flow vortices in the engine cylinder, as indicated in FIG. 3. Fuel is injected from the injection nozzle 201, as indicated in FIG. 4, at about maximum compression.

Additional openings 150 may be provided in the wall of the engine cylinder 101. These openings, one of which is presently shown closed by a cover 151, may be used to apply compressed air for moving the pistons apart if necessary. (when the engine is inoperative.) or to connect a pressure gauge for research or experimental purposes, or to provide an alternative means for fuel injection or fuel injection timing, or to admit air when the pistons are being set in a starting position, as will be explained hereinafter.

The engine exhaust system comprises a casing 152 providing a storage space 153, which communicates with the exhaust ports 103 via a manifold or passage 154 in the engine cylinder. A casing 155 bolted to the bottom end of the casing 152 provides an internal space 156 which communicates with the storage space 153 by way of ports 157. An intake tube 158 connected to the upper end of the casing 155 projects upwards in alignment with the passage 154, the latter being spaced from the end of the intake tube. The casing 155 also provides an internal cylinder portion containing a spool valve 160, which is biassed upwardly by a spring shown diagrammatically at 159. In operation of the engine, when the exhaust ports 103 are uncovered by the piston 104R towards the end of a. compression stroke, the products of combustion enter the storage space 153 and impinge upon the intake tube 158. If the kinetic energy of the exhaust gases is relatively low, the spool valve 160 remains in its upper position and the exhaust gases pass to a silencer (not shown) via ducting 161.

However, if the kinetic energy of the exhaust gases is sufficiently high, the spool valve 160 is displaced downwards to cover the ports 157; in this case the gases in the storage space 153, being of increased pressure, pass through a duct 162 to an exhaust turbine, the latter being combined with the air compressor 146. The spent gases are finally exhausted via a pipe 163 and a silencer (not shown).

Each of the engine pistons is formed with a circumferential annular groove 169 having bevelled sides, into which a plunger 170 having a correspondingly bevelled end may be forced, in order to lock the pistons against movement when the engine is not running. In FIG. 1, the plungers 170 are shown neither fully in nor fully out, but are shown for illustration in an intermediate position. Each plunger 170 is normally held out of the respective annular groove 169 when the engine is running, by a *U" spring 171 which engages a grooved portion 172 of the plunger. When required, the plungers 170 are pressed into their operative, piston-holding positions by actuating pistons 173; the latter are slidable' in cylinders 174 and are actuated by hydraulic pressure applied via oil connections 175 when the pistons 104L and 104R are near the ends of their expansion strokes. The plungers 170 hold the pistons 104L and 104R approximately in the position shown in FIG. 1, against the forces exerted by the rams 113, the latter being urged by hydraulic pressure from the accumulators 108. The forward speed of each actuating piston 173 is controlled by an orifice in a plate valve 167; the parts are designed to permit comparatively free flow in the reverse direction when the engine is being started. A light spring 168 holds the valve plate normally forward.

The output of the engine is a flow of pressurized liquid. The amplitude and frequency at which the engine pistons 104L and 104R reciprocate are variable, depending upon the power that is being developed. The positions at which the pistons momentarily stop at the end of the compression stroke are largely determined by the initial momentum of the pistons and the pressure of the initial cylinder air charge. The positions at which the pistons momentarily stop at the end of the expansion stroke are determined by the momentum that they have gained from the energy of combustion, the cyclic range of oil pressures, and the rate of flow of the hydraulic liquid. To maintain an approximately constant compression ratio in the engine cylinder 101, more energy will be required at high intake air pressures than at low intake air pressures; however, the same invariable volume of oil, determined by the stroke of each piston 123 of the constant displacement hydraulic accumulators, as it reciprocates between the limits of its movement, will always be set aside for the return strokes of the engine pistons. Therefore the average pressure of this oil must be altered as required in accordance with the pressure of the intake air. It can easily be shown that when the average oil pressure in each of the constant displacement accumulators 108 is low, (to accommodate low intake air pressure,) the average compression speeds of the pistons 104L and 104R will also be low and the time taken to effect the compression strokes will be correspondingly long. Conversely, when the average oil pressure in each of the constant displacement accumulators 108 is high, the speeds of the pistons 104L and 104R will be high, so that the time to effect the compression strokes will be correspondingly short. The same factors apply to the speeds of the pistons on the-expansion strokes, so that the time to complete the strokes will be an inverse function of the energy developed. The net result is that the rate of reciprocation of the engine pistons will be low at low power outputs and high at high power outputs. Power Unit Operation With the stop plungers 170 completely retracted, the engine'pistons 104L and 104R and the pump pistons 113 are initially in the positions shown in FIG. 1 with the engine pistons moving towards one another; the engine pistons 104L and 104R and the pump pistons 113 together have sufficient momentum to give an air'compression ratio of, say :1 or higher. Subsequently to this initial stage the oil inlet plate valves 133 open. The air inlet valves 141 are already open and the air delivery valves 143 are closed, so that air enters the air compression spaces in the engine cylinder lying between the engine pistons and the bulkheads 110. The pump pistons 113 also move out from the pump chambers 112, under the action of their momentum and the pressurized fluid passing through the .inlet oil valves 133. At about the position when the momentum of the pump pistons, together with the momentum of the engine pistons 104L and 104R, is spent'in compressing the air charge in thecylinder 101, fuel is injected into the cylinder by the fuel injector 200; the gas charge temperature and the relative pressure then rises and the engine pistons are caused to accelerate away from each other, performing their working or expansion stroke.

The one-way plate valves 133 then close and the pump pistons 113 are forced inwards by pistons 104, first to charge the accumulators 108, which are adjusted so as to yield at less pressure than the pistons 136 of the variable displacement accumulators 109. When the pistons 123 hav completed their strokes, which are terminated by the abutment of these pistons against the lower ends ofthe sleeves 121, the pressure in the pump chambers 112 continues to rise and the one-way plate valves 134 open against the pressure in the oil spaces 139 of the variable displacement accumulators 109. During normal running of the unit, the pistons l36 are at a higher position than that shown in the drawings, depending upon the pressure required. The surge of oil on each pumping stroke, after first displacing the pis-v tons 123 in the constant displacement accumulators 108, is absorbed in urging the pistons 136 inwards against the pressure of gas in the gas spaces 137, but the oil is continually leaving at a more moderate velocity through the outlet ports 140. When the momentum of the engine pistons 104L'and 104R is spent, the pistons stop and the high speed plate valves 134 close. The pistons 123 are subjected to pressure from the gas above them and still maintain a substantial pressure on I the oil in the pump chambers 112; this pump chamber pressure acts on the pump pistons 113 and accelerates the engine pistons 104L and 104R back along another compression stroke as already described. During the time in which the pistons 104L and 104R are moving outwards on the expansion stroke, the pressure in the air compression spaces 117 is increasing, and when this pressure exceeds the pressure above the air delivery valves 143, the latter open to admit this air charge.

Outlets 176 from the pump chambers 112 are connected to pipes 515L, 515R (FIG. 41), as will be de scribed hereinafter. A peripheral groove 119 is incorporated in the piston-rod bearing of each bulkhead as part of a means to ensure synchronisation of the engine pistons 104L and 104R, as will also be explained here inafter. By these means continuous running of the engine is achieved.

THE FUEL INJECTOR the injection means in accordance with the air pressure within the engine cylinder so as to ensure that the predetermined quantities of fuel are injected into the cylinder at appropriate times in relation to the combustion cycle of the engine. I

The injection means comprises, basically, a fuel injection nozzle, a valve controlled supply chamber located behind the injection nozzle, means for admitting fuel to the fuel supply chamber, and a fuel piston actuated by said actuator means to compress the fuel in the supply chamber and to expel the fuel therefrom to the engine cylinder via the injection nozzle. The actuator means includes a spring-loaded shuttle valve arranged to move towards one or other of two limit positions in accordance with the gas pressure in the engine cylinder, and means for supplying pressurized hydraulic fluid to actuate the fuel piston in accordance with the position of the shuttle valve; the pressurized hydraulic fluid is fed from a chamber housing a free piston which is urged in a direction to expel hydraulicfluid from the chamber, explusion of fluid from the chamber being controlled by the shuttle valve, which is arranged to cover and uncover a port leading to the chamber. Arrangement and Mechanical Construction The fuel injector is illustrated in detail in FIGS. 18 to 26 of the drawings, of which FIG. 18 best shows the interrelationship of its working parts. Reference will now be made to these figures in particular.

In FIG. 18 is shown a portion of the engine cylinder 101, and portions of the engine pistons 104L and 104R which define a combustion space S in the engine cylinder, into which fuel is injected by the fuel injector. The fuel injector itself is incorporated in a metal body 202, which is machined to provide a number of internal passages and bores as hereinafter described, and which houses the essential elements of the injection means and the actuator means referred to above.

The metal body 202 is formed with a stepped cylindrical bore 203,- at the upper end of which is an assembly consisting of the injection nozzle itself 201, a spring-loaded valve 204 having a valve seat 205, a spacer ring 206, and another spacer ring 205A which may be of relativelysoft metal such as mild steel, those parts being clamped and retained in position by an adaptor 207 which is screwed into the threaded upper end of the cylindrical bore 203. The adaptor 207 is located in a. passage in the 'wall of the engine cylinder 10], to which the fuel injector body 202 is suitably connected, a sealing ring 208 being located so as to prevent leakage of gases from the engine cylinder.

Located within the cylindrical bore 203 is a cylindrical barrel 209, the barrel being a tight leak-free fit within the bore. A piston 21-0 having trunk extension 

1. A power unit comprising a free piston engine having a reciprocatory power piston and a hydraulic pump piston coacting therewith, the pump piston working in a pump chamber having an inlet opening and a delivery opening, a constant displacement hydraulic accumulator connected to the pump chamber to receive hydraulic fluid therefrom during the power stroke of the engine and to return the fluid thereto during the compression stroke of the engine, a first one-way valve in the inlet opening and operable in response to the discharge of the accumulator to a predetermined minimum level to admit hydraulic fluid to the pump chamber during the compression stroke of the engine, and a second one-way valve in the delivery opening and operable in response to the charging of the accumulator to a predetermined maximum level to deliver hydraulic fluid under pressure from the pump chamber during the power stroke of the engine.
 2. A power unit according to claim 1, including a second hydraulic accumulator connected to the delivery opening of the pump chamber to receive fluid under pressure therefrom in accordance with the opening of the second one-way valves.
 3. A power unit according to claim 1, wherein the one-way valves are spring-loaded, fluid pressureoperated, plate valves.
 4. A power unit according to claim 1, the engine cylinder providing inlet and exhaust ports communicating with respective inlet and exhaust manifolds for admitting combustion air to, and exhausting combustion gases from the cylinder, the engine cylinder further providing an air compression space located behind the engine piston, the air compression space having a valve-controlled air inlet and a valve-controlled air outlet and duct means connecting said air outlet with the inlet manifold.
 5. A power unit according to claim 1, wherein the constant displacement hydraulic accumulator comprises a cylinder having a piston slidable therein between first and second limit positions which determine said minimum and maximum charge levels of the accumulator.
 6. A power unit according to claim 1, wherein the engine is a compression-ignition engine.
 7. A power unit according to claim 1, including a supply manifold connected to said delivery opening to receive pressurized hydraulic fluid from said pump chamber, and valve means for controlling the delivery of fluid to the supply manifold, said valve means being responsive to the operating pressure of the constant displacement accumulator and operable to prevent the delivery of fluid to the supply manifold save when the pressure in the supply manifold is greater than said operating pressure.
 8. A power unit comprising an internal combustion engine having a pair of opposed free pistons working in a common cylinder, a pair of pump units connected to the cylinder at opposite ends thereof, each pump unit comprising a pump piston coacting with a respective one of the engine pistons and working in a pump chamber having an inlet opening and a delivery opening, a pair of constant displacement hydraulic accumulators connected respectively to the pump chambers to receive hydraulic fluid under pressure therefrom during the power stroke of the engine pistons and to return the fluid thereto during the compression stroke of the engine pistons, each pumping unit including a first one-way valve in the inlet opening of the pump chamber and operable in response to the discharge of the respective hydraulic accumulator to a predetermined minimum level to admit hydraulic fluid to the pump chamber during the compression stroke of the engine pistons, and a second one-way valve in the delivery opening of the pump chamber and operable in response to the charging of the respective hydraulic accumulator to a predetermined maximum level to deliver fluid under pressure from the pump chamber during the power stroke of the engine pistons.
 9. A power unit according to claim 8, including a second pair of hydraulic accumulators connected respectively to the delivery openings of the pump chambers to receive fluid under pressure therefrom in accordance with the opening of said second one-way valves.
 10. A power unit according to claim 8, wherein the one-way valves are spring-loaded, fluid pressureoperated, plate valves.
 11. A power unit according to claim 8, wherein each of the constant displacement hydraulic accumulators comprises a cylinder having a piston which is slidable tHerein between first and second limit positions which determine said minimum and maximum charge levels of the accumulator.
 12. A power unit according to claim 8, wherein the engine cylinder is oriented horizontally and wherein each constant displacement hydraulic accumulator comprises a cylinder having a stepped internal surface, a coaxial cylindrical sleeve, having a free end spaced from said internal surface, a first piston slidable within said sleeve and defining therein a space for hydraulic fluid, a second piston slidable within the accumulator cylinder and defining with the first piston a space for elastic fluid, said second piston being slidable between a first limit position determined by said stepped internal surface and a second limit position determined by the free end of said sleeve.
 13. A power unit according to claim 8, including a supply manifold connected to said delivery openings to receive pressurized hydraulic fluid from said pump chambers, and valve means for controlling the delivery of fluid to the supply manifold, said valve means being responsive to the operating pressures of the constant displacement accumulators, and operable to prevent the delivery of fluid to the supply manifold save when the pressure in the supply manifold is greater than said operating pressures.
 14. A power unit according to claim 8, including a pair of further hydraulic accumulators disposed adjacent to, and connected to, said inlet openings of the pump chambers for supplying hydraulic fluid thereto via the first one-way valves, each of said further hydraulic accumulators being connected to a manifold for receiving hydraulic fluid therefrom.
 15. A power unit according to claim 14, including a sump system for the storage of hydraulic fluid under pressure, duct means connecting said further hydraulic accumulators to the sump system and valve means responsive to the pressure in the sump system for controlling the supply of hydraulic fluid from the sump system to said further accumulators.
 16. A power unit according to claim 15, wherein the sump system includes a storage cylinder containing a free piston which determines a storage space of variable volume, the free piston being slidable over a fixed piston and defining therewith an internal chamber, and means for supplying pressurized hydraulic fluid to said internal chamber to maintain said storage space at constant pressure.
 17. A power unit according to claim 16 including a valve connected to said storage space, the valve being responsive to the pressure in said storage space and operable to control the supply of hydraulic fluid to said internal chamber.
 18. A power unit according to claim 8, wherein the engine is a compression-ignition engine.
 19. A power unit as claimed in claim 8, the engine cylinder providing inlet ports and exhaust ports communicating with respective inlet and exhaust manifolds for admitting combustion air to, and exhausting combustion gases from, the cylinder, the engine cylinder further providing first and second air compression spaces located respectively behind the engine pistons, each air compression space having a valve controlled air inlet and a valve controlled air outlet, and duct means connecting said air outlets with the inlet manifold.
 20. A power unit as claimed in claim 19, including an air compressor for supplying compressed air to said valve controlled air inlets, a turbine drivingly coupled to the air compressor, means connected to the exhaust manifold and to the turbine for supplying exhaust gases to the turbine, and valve means responsive to the pressure of the exhaust gases for controlling the supply of said gases to the turbine.
 21. A power unit according to claim 20, including a spool valve connected between said storage space and each of said spaces for hydraulic fluid defined by said first pistons of the constant displacement accumulators, the spool valve being responsive to gas pressure in said inlet manifold and operable to control the supply of pressuRized hydraulic fluid to and from said defined spaces chamber or from the auxiliary chamber to said outlet, respectively.
 22. A power unit as claimed in claim 8, wherein each pump piston is a hollow ram defining an internal oil space and having an oil flow restrictor at one end, said oil flow restrictor registering with a lubricant channel in the respective free piston, the ram housing a free plunger which is spring-biassed towards the oil flow restrictor, the plunger being operable to admit oil into the lubricant channel in response to reversing movements of the ram.
 23. A power unit as claimed in claim 8, wherein each of the engine pistons is formed with a circumferential groove which, in one position of the piston, registers with a plunger which is radially movable from a retracted position to a position in which it engages in the groove to lock the piston, the plunger being normally retained by a spring in the retracted position and being urged into its engaging position by hydraulic means, the unit including means for returning the plungers to the retracted positions to permit the starting of the engine.
 24. In combination with a power unit as claimed in claim 8, the engine being a compression-ignition engine, a valve operated fuel injector actuated by engine cylinder pressure to control the injection of measured quantities of fuel into the engine cylinder.
 25. The combination claimed in claim 24, wherein the fuel injector comprises a fuel injection nozzle, a valve-controlled supply chamber located behind the injection nozzle for storing a quantity of fuel, means for admitting fuel to the supply chamber, a fuel piston operating in the supply chamber for compressing the fuel stored therein and expelling the fuel from the supply chamber to the engine cylinder via the fuel injection nozzle, and hydraulic actuator means responsive to engine cylinder pressure for actuating the fuel piston.
 26. The combination claimed in claim 25, wherein said actuator means includes a spring-loaded shuttle valve arranged to move towards one or other of two limit positions in accordance with the gas pressure in the engine cylinder, and means for supplying pressurized hydraulic fluid to actuate the fuel piston in accordance with the position of the shuttle valve.
 27. The combination claimed in claim 25, wherein said actuator means includes a spring-loaded shuttle valve slidable in a valve cylinder, the valve cylinder having an inlet for pressurized hydraulic fluid, an outlet communicating with the fuel piston, and a port communicating with an auxiliary chamber wherein a free piston is biassed in a direction to expel fluid from the auxiliary chamber via said communicating port, the shuttle valve being movable between two limit positions in accordance with the gas pressure in the engine cylinder for determining a flow of pressurized hydraulic fluid from said inlet to the auxiliary chamber or from the auxiliary chamber to said outlet, respectively.
 28. In combination with a power unit as claimed in claim 8, first and second pressure responsive means connected respectively to said pump chambers for producing first and second pressure signals, each said pressure responsive means cooperating with a respective one of said pump pistons and being operative for a predetermined fraction of the respective piston stroke, a differential valve responsive to said first and second pressure signals and continuously operable in accordance with any difference therebetween, and hydraulic means responsive to the operation of said differential valve for controlling said constant displacement hydraulic accumulators differentially whereby to reduce any difference between said signals and so maintain synchronous operation of the opposed engine pistons.
 29. The combination claimed in claim 28, wherein each of the pump chambers is formed with an internal groove extending transversely to the direction of movement of the respective pump piston, and wherein each of said pressure responsive means comprises a duct extending from a respective one of said grooves to the differential valve, each groove being positioned in its pump chamber so as to be uncovered by the pump piston for a predetermined fraction of its stroke.
 30. In combination: i. a compression ignition engine having a pair of opposed free pistons working in a common cylinder, ii. the cylinder providing inlet ports and exhaust ports communicating with respective inlet and exhaust manifolds for admitting combustion air to, and exhausting combustion gases from, the cylinder, iii. the cylinder further providing first and second air compression spaces located respectively behind the engine pistons, each air compression space having a valve controlled air inlet and a valve controlled air outlet, iv. a pair of pump units connected to the engine cylinder at opposite ends thereof, v. each pump unit comprising a pump piston coacting with a respective one of the engine pistons and working in a pump chamber having an inlet opening and a delivery opening, vi. a pair of constant displacement hydraulic accumulators connected respectively to the pump chambers to receive hydraulic fluid therefrom during the power stroke of the engine pistons and to return the fluid thereto during the compression strike of the engine pistons, vii. a first pair of spring-loaded, fluid pressure-operated, one-way valves lcoated respectively in said inlet openings of the pump chambers and operable to admit hydraulic fluid thereto during the compression strokes of the engine pistons, viii. a second pair of spring-loaded, fluid pressure-operated, one-way valves located respectively in said delivery openings of the pump chambers and operable to deliver hydraulic fluid therefrom during the power strokes of the engine pistons, and ix. a pair of high pressure hydraulic accumulators connected respectively to the delivery openings of the pump chambers to receive hydraulic fluid therefrom in accordance with the opening of said second pair of one-way valves, each of said high pressure hydraulic accumulators providing means for delivering fluid under pressure to a hydraulic power transmission system. 